1. Field of the Invention
The present invention relates to an optical attenuator, and particularly to a variable optical attenuator having an electrical controlling circuit.
2. Description of the Related Art
Optical attenuators are used to optimize the optical power of signals at key points in optical communications networks. For example, in networks having Erbium Doped Fiber Amplifiers (EDFAs), optical attenuators are used between stages of EDFAs to provide constant gain. In Wavelength Division Multiplexer (WDM) systems, optical attenuators are used to adjust optical power of xe2x80x9caddedxe2x80x9d laser signals to match the signals strength of other channels within the network. Optical attenuators can also be used to set signal strength within the range of a particular receiver.
Known methods to obtain a variable optical attenuator include coating a filter element with an attenuation layer having a variable density, and bending optical fibers to get a given attenuation. A variable optical attenuator can also be obtained by changing a distance between a reflector and an input port or an output port.
U.S. Pat. No. 5,745,634 discloses a voltage controlled attenuator comprising a first lens for receiving incoming optical signals, a second lens for outputting the attenuated optical signals, an optical power detector and a controllable attenuating element. The optical power detector monitors the intensity of the attenuated optical signals, and the controllable attenuating element varies the attenuation of the outputting optical signals in response to electrical signals from the optical power detector. A weakness of this prior art arrangement is that the optical power detector is separate from the second lens. The attenuated optical signals reflected by an input face of the second lens have to travel a distance to the optical power detector, which wastes a portion of the reflected optical signals. An arrangement which utilizes the reflected signals more efficiently is desired.
An object of the present invention is to provide an optical attenuator which controllably attenuates optical signals by using an electrical controlling circuit.
Another object of the present invention is to provide an optical attenuator which accurately and flexibly controls attenuation of optical signals by using an optical splitter and two sampling detectors.
An optical attenuator in accordance with the present invention comprises: an optical splitter, a collimator, an input and an output detectors, a first and a second reflectors, an attenuating element and a driving device. The splitter includes a ferrule and a GRIN (graded index) lens. Input optical signals are transmitted from an input fiber through the splitter and are then directed to the first reflector. The optical signals reflected by the first reflector pass through the attenuation element and subsequently are reflected to a collimator by the second reflector, and are then directed to an output fiber. The input and output detectors detect the intensity of input optical signals and output optical signals and a control circuitry calculates the attenuation ratio. The driving device then drives the attenuating element in response to the attenuation ratio.